Petroleum recovery techniques generally divide into three categories, namely, primary, secondary, and tertiary recovery. Primary recovery merely involves drilling to the oil-bearing formation and recovering the petroleum using the natural pressure of the well to force the petroleum through a hollow pipe to the ground. Secondary recovery techniques typically involve injecting water into the well to force a mixture of petroleum and water through the well pipe to the surface.
When primary and secondary recovery techniques have ceased to remove significant amounts of petroleum, the well is defined as a "spent" well. Nevertheless, the amount of oil remaining in spent wells is, on the average, twice as large as that which has already been removed. At least some of this oil may be recovered by tertiary techniques. Thus, theoretically, we have reserves in depleted oil fields that are twice as large as the total amount of petroleum mined in this country since the industry began about a century ago. While, until recently, the employment of tertiary recovery techniques has not been economically feasible, rising world prices for the commodity has spurred the development of this sector of oil recovery technology.
Most tertiary oil recovery techniques fall into three areas, namely, thermal, miscible, and chemical. Chemical flooding generally involves the use of a detergent-like chemical which is pushed through the formation to dislodge the oil from pores in the rock by reducing the interfacial tension between water and oil and drive it to collection points from which it can be recovered. Miscible systems generally involve pumping a solvent into the formation where it mixes with the oil, forming an oil/solvent solution. This solution, because of its lower viscosity, may be pumped to the surface. In all of these cases, the mixture of oil and other substances is separated, and the oil recovered. However, neither of these systems has seen widespread use in view of the relatively large costs associated with the chemicals and processing involved.
Generally, thermal recovery techniques involve the generation of steam at the surface, and pumping of that steam and compressed air through the well pipe to the oil reservoir. The heat from the steam has the effect of decreasing the viscosity of the heavy crude in the formation. The steam under pressure then travels into the formation exerting pressure on the oil, as well as, to some extent, heating it. Ultimately, this oil is driven into perforations in the lower portion of the well pipe, thus passing into the well pipe and being drawn to the surface.
However, steam systems suffer from a number of problems. Specifically, the amount of oil consumed to generate the steam comprises a substantial portion of the oil recovered through the use of the technique. The effectiveness of the technique is hampered by the cooling and condensation of the steam on its way down to the bottom of the well. Inefficiency of the system is compounded by the loss of a substantial proportion of heat through the outer walls of the well pipe as the steam travels down to the oil-bearing formation with attendant decreases in temperature and pressure. Nevertheless, thermal recovery techniques appear to be the best hope for obtaining oil from "spent" formations.